Tiny wasp could be answer to stink bug problem

The brown marmorated stink bug presents a serious threat to important food crops in New Zealand and around the world. But, as the University of Auckland's Tom Saunders explains, there is a type of tiny wasp that could help stop it.

Stink bugs don't generally weigh heavily on the minds of most. We normally only notice these peanut-sized insects when they release their pungent defensive odours when disturbed. But many New Zealand producers are sitting up and taking notice of a new pest on the block: Halyomorpha halys, commonly known as the brown marmorated stink bug.

For the last 20 years this voracious plant pest has been on the march. It was first detected in the United States during the 90s, and has since swept through 44 US states and four Canadian provinces. The bugs entered Europe in 2007 and are now established in at least 10 countries there. The first southern hemisphere detection, in Chile, was confirmed in March of last year.

This does not bode well for New Zealand.

Interceptions at the border have skyrocketed in recent years. Since September 2017, there have been more than 157 separate incidents involving over 900 adult stink bugs. Australia, too, is feeling the pain: just this month several stink bugs were spotted in a container of furniture from Italy, putting Australian biosecurity personnel on high alert.

New Zealand's Ministry of Primary Industries is working alongside those in the horticulture sector here to batten down the hatches. The brown marmorated stink bug was recently classified as a top biosecurity threat. Strict new treatment requirements are being enforced on all sea containers arriving from Italy; and industry groups are spreading the word to prepare for the coming invasion.

So what would happen if the brown marmorated stink bug did establish here?

It would start by eating crops that we like to eat, especially pipfruit, stonefruit, kiwifruit, berries, citrus, corn, and grapes. The bugs use their piercing-sucking mouthparts to suck juices out of plants and fruits, causing unpleasant marks and opening the way for secondary infection.

American apple growers suffered around US$37million in lost fruit within the single 2010 growing season.

Threatened crops in New Zealand are valued at over $4 billion per annum. Horticulture New Zealand has estimated the establishment of these bugs would lead to 26 percent lower yields for 18 priority crops if left unmanaged. Then there's the threat to our wine industry. It only takes a few stink bugs per batch to produce a revolting taint in the finished product.

But feeding damage isn't the only problem. When temperatures drop, adult stink bugs seek shelter to overwinter. This causes massive nuisance problems: Residents in some US towns are forced to literally sweep carpets of stink bugs out of their homes, garages, and vehicles. Dealing with extermination and clean-up hits people in the wallet, but also in lost productivity, stress, and allergic reactions to the smelly compounds.

The tiny samurai wasp, Trissolcus japonicus, may be used as a biological control agent against BMSB in New Zealand. Photo: Oregon State University CC-BY-SA 2.0

And just in case you were thinking about the broad spectrum pesticides that are effective against brown marmorated stink bug overseas, think again. A decent chunk of New Zealand's trading reputation is made on our proud tradition of residue-free produce.

As part of the Better Border Biosecurity Collaboration, scientists from the University of Auckland and Plant & Food Research are working on a method of control that may involve a tiny Asian species of wasp. Trissolcus japonicus, nicknamed the 'Samurai Wasp', attacks the brown marmorated stink bug eggs in their native range. The three millimetre wasp is an egg parasitoid which means it lays its own eggs into the stink bug eggs, killing them in the process.

There are thought to be hundreds of thousands of species of parasitoid wasp worldwide, all different shapes and colours, and attacking all sorts of insect eggs and larvae.

In New Zealand we have an estimated 2000-3000 native wasp species. The four or five species of introduced wasps that live in colonies and sting people get all the headlines, but our native wasps are nothing like these.

Added to this is a handful of species deliberately introduced to control agricultural pests. Many of these are remarkably successful. For example, a parasitoid wasp from Kazakhstan, Cotesia kazak, was introduced to control Helicoverpa armigera, or Tomato Fruitworm. The larvae of this moth species cause severe damage to tomato crops. Growers in New Zealand were forced to apply pesticides to prevent the disappearance of their crop. But the introduction of the parasitoid caused a 95 percent reduction in pesticide use, saving the growers time and money, and contributing to more sustainable tomato production.

Scientists from the B3 Collaboration are now trying to figure out whether or not introducing the samurai wasp would lead to a similar outcome in the fight against the brown marmorated stink bug when it gets into the country.

In order to do this, we have to measure the risk of the parasitoid attacking non-target species – and in this case, the parasitoid is restricted to stink bugs. We are lab rearing all of the stink bug species found in New Zealand so we can test how attractive they are to the wasp.

Results will be looked at by the Environmental Protection Agency and used to decide whether or not to introduce the samurai wasp, if and when BMSB is found in New Zealand.

Biological control offers some great tools for managing pests in a more environmentally-friendly way. Modern biocontrol methods emphasise the importance of determining risk accurately. Releases of new organisms in New Zealand are now only possible after extensive study of the potential risks and benefits.

Pest insects throw up some tricky challenges and kiwi scientists are world famous for meeting that challenge with perseverance and ingenuity. But it’s not just the scientists – as a biosecurity team of 4.7 million living here in New Zealand, we all have a part to play in protecting our economy, livelihoods and health from incoming pests and disease.

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